by Dr. April Eryou
A hemianopsia is a loss of vision in half of the visual field occurring from an insult to the visual pathway from the retina to the visual cortex. There are various types of hemianopsias depending on the area of the brain that is affected. Bitemporal hemianopsias are caused by damage to the optic chiasma. If the hemianopsia is occurs first or is deeper in the upper quadrants, then the optic chiasma is being compressed from below. Possible diagnoses would be: pituitary adenoma, nasopharyngeal carcinoma and sphenoid sinus mucocele. If the hemianopsia first occurs or is deeper in the lower quadrant, then the optic chiasma is being compressed from above. Possible diagnoses would be: craniopharyngioma or third ventricular tumor. Binasal hemianopsias are uncommon. In the literature, they have been associated with bilateral internal carotid aneurysms, hydrocephalus, neurosyphilis, congenital defects, intracranial mass lesions and increased intra-cranial pressure. Homonymous hemianopsias (HH) are when either the right or left visual fields in both eyes is affected. The majority of HH, 52-70% are caused by strokes, with 8-10% of stroke victims having a HH, less common causes of HH include brain injury: 14% and tumors: 11%5. If the HH is incongruous, then the lesion is in the optic tract close to the chiasma; sudden onset of symptoms would suggest a vascular cause and a gradual onset would suggest a tumour in the area. If the HH is congruous, then the problem is in the occipital lobe. In these cases, if the macula is affected then the problem also involves the pole of the calcarine cortex and the problem is usually due to a tumor. When the macula is spared, the most common cause is vascular, as there is a dual blood supply to the pole of the calcarine cortex. Superior and inferior hemianopias are more commonly seen in problems affecting the optic nerve and would likely present asymmetrically. Rarely, they can also be from intracranial problems. If there are bilateral lesions in the occipital lobe either above or below the calcarine fissure then the patient would also have a hemianopia. Bilateral superior altitudinal hemianopia’s have also been noted in patients with bilateral ischemia in the optic radiations.
So long as the patient does not have concurrent neglect, patients who have a hemianopia not related to a congenital cause, will be aware of the vision loss. Visual field testing: Goldman, Tangent or Humphrey 24-2 would solidify the diagnosis of a hemianopsia and that would direct further imaging/testing based on the specific characteristics (as mentioned above) of the defect. Given the extent of visual field loss in a hemianopia, practitioners would notice the visual field loss during the optometric exams. When history taking, patients will complaint of difficulties with ambulation, they will tend to miss or knock into things on the affected side and reading will be difficult because saccades across midline will be impaired. When reading the English language, left side hemianopias will cause challenges with return saccades and right side hemianopias will cause challenges moving the eyes along the text. A right sided hemianopia is more detrimental to left to right reading than a left sided hemianopia. When testing acuities, the patient may have difficulty reading all of the letter on a row when shown the full eye chart. When testing ocular motility, the patient will have difficulty doing saccades across the midline. Patients may have challenges using only their eyes when doing pursuit testing. During confrontational visual field testing, because the field loss is extensive, the location of the field loss will be apparent. A full neuro-optometric examination including perceptual testing would be suggested for these patients because the degree of neurological insult will likely impact more than just the visual field loss.
Treatment options for patients with hemianopia visual field loss involves work with a multidisciplinary team. Physical therapist can help the patient if they are experiencing balance difficulties. Occupational therapy can help the patient adjust their home and work activities and learn effective ways to navigate around space. Psychological support can help the patient deal with the changes in their function and ensure they have social and emotional support. Neuro-optometric rehabilitation and low vision experts can help the patient learn effective scanning techniques to improve movement through the environment, they can enhance a patient’s awareness to the remaining visual field and they can help the patient learn techniques so that they can resume reading. Optometrists and low vision experts can also prescribe field enhancing prisms. There are several different prisms systems that have been developed for patients with lateral HH field loss. A commonly used one is the peli system. On the eye closer to the field loss, 40 pd Fresnel prisms, with the base in the direction of the field loss, is put on the top and bottom of the lens leaving the central part free of prism to ensure there is no central diplopia. This can provide an increase in field of 20 degrees. The downside to Peli prism correction is that it can be difficult for patients to adapt to due to challenges going down stairs, perceptions that objects jump into the field of view and potential that if Fresnel prisms are used, the optical quality can deteriorate over time requiring frequent replacement5. Optometrists could also provide the patient with yoked prism glasses to help the patient attend to the missing field. Yoked prisms (with the base in the direction of the missing field) are used to help centre the visual field.
5. Goodwin D. Homonymous hemianopia: challenges and solution. Clin Ophthalmology.2004;8: 1919-1927